At present, a cellular system may use a multicarrier system. Such systems are known in the art, and may comply with the DVB-T standards in Europe, or MMDS (Multichannel Multipoint Distribution Service) in U.S.A. These systems use the Orthogonal Frequency Division Multiplexing (OFDM) method, wherein symbols are transmitted in parallel by modulating a large number of orthogonal subcarriers.
A base station may be allocated, for example, 1700 out of 2000 subcarriers. The subcarriers may span a given frequency range, while being equidistantly located in the frequency domain.
The problem is how to allocate the various subcarriers to subscribers so as to minimize interference between users.
To illustrate such a problem: a base station is allocated, for example, a frequency range comprising 1600 subcarriers, divided into 64 groups of 25 subcarriers each.
A first user is allocated a spectrum comprising a plurality of subcarriers, one in each of the 64 groups.
A second user is allocated a spectrum comprising a plurality of subcarriers, a different one in each of the 64 groups.
Preferably, all the subcarriers allocated to the second user are different than those allocated to the first user.
Additional users are allocated other subcarriers.
One problem in the abovedetailed scheme is a possible collision between users, when two users are allocated the same subcarrier. Such collisions are undesirable, as they may generate errors in communication, which may possibly increase the Bit Error Rate (BER) or slow down the communication channel.
Collisions may even occur if users are allocated different subcarriers. Thus, if there is a frequency deviation in the transmission of a first user and there are some subcarriers of that user which are close in frequency to some subcarriers of another user, then collisions may occur.
A collision occurs as the spectrum of that first user slides in frequency, to partially overlap with the spectrum of the second user.
Such frequency deviations may occur, for example, when one of the users is in motion, because of the Doppler effect.
The need for the use of smart subcarrier allocation may come from the frequency deviation that a user can suffer and its effect on other users. Allocating to users with all of their carriers allocated one adjunct to the other can cause, in a frequency deviation scenario a destruction of the deviated user and the adjunct one.
As the number of colliding subcarriers increases, the undesired disruption is more detrimental to communications.
Therefore, there is a problem of subcarriers allocation to users so as to reduce this undesirable effect.
The problem is further complicated in a multiuser cellular system, where there is a plurality of users communicating to a base station. Each user is allocated a plurality of subcarriers, some of which may possibly be close in frequency to other user's subcarriers.
This is a complex, multidimensional problem that may be difficult to solve.
The above description relates to communications between one base station and users in that cell.
In real life, the problem is further aggravated because of interference between adjacent cells. Thus, a base station in one cell may receive interfering signals (subcarriers) from users in a nearby cell.
It is an objective of the present invention to address the problems of the allocation of subcarriers in a multicarrier cellular system.